U.S. Pat. No. 4,948,458 to Ogle describes an apparatus for producing planar plasmas that can operate over a wide pressure range. The apparatus is a vacuum chamber having a dielectric window or shield in one wall of the chamber. A planar coil, outside of the chamber and proximate to the dielectric shield, and an RF source is coupled to the coil. The chamber is also fitted with a port for the inlet of plasma precursor gases into the chamber, and a port for ingress and egress of a substrate to be processed, as well as a support for the substrate parallel to the dielectric window. When an RF current is applied to the coil, a changing magnetic field is induced which extends inside the chamber through the dielectric shield, inducing a circular flow of electrons within the processing region of the chamber. This induced circular electric field is substantially in a plane parallel to the planar coil, which reduces the transfer of kinetic energy in the non-planar direction. The substrate to be etched is also mounted in the direction of the plane of the plasma and thus the velocity component of charged particles in the non-planar direction with respect to the substrate during processing is minimized, and the treatment on the substrate is generally limited to the chemical interaction of the plasma species with the substrate, except in the case where RF bias is applied to the substrate with respect to a grounded electrode or chamber. The entire disclosure of U.S. Pat. No. 4,948,458 is incorporated herein by reference.
The above plasma reactor is useful for etching materials such as aluminum, but it has limitations with respect to etching oxides such as silicon oxide, which are required in the manufacture of semiconductor devices. Silicon oxide films and layers, for example, are applied to various substrates during the manufacture of silicon devices, including silicon, metal layers, silicon nitride and the like. Typically a photoresist is deposited over the silicon oxide layer to be etched and patterned, and the silicon oxide etched with a fluorohydrocarbon gas such as CF.sub.4, C.sub.2 F.sub.6, C.sub.3 F.sub.8, CHF.sub.3 and the like. For example, a via in a silicon oxide layer over polysilicon may be etched and the via later filled in with a conductor to make contact between the underlying polysilicon and a conductive layer overlying the silicon oxide. In order to fill in vias, which are becoming smaller and deeper, the etch process has several stringent requirements; the sidewalls of the vias must be straight (anisotropic etch) and the etching must be selective with respect to the overlying photoresist layer and the underlying material, i.e., the etch process must etch the silicon oxide layer at least at a faster rate than overlying and underlying layers, and preferably the selectivity should be higher than about 10:1. For other semiconductor devices and arrays, large and small features are present and must be etched at the same time, requiring that large and small features in the same material, e.g., silicon oxide, be etched at the same rate, i.e., without microloading. Microloading for purposes herein is defined as ##EQU1## Still further, since silicon oxide layers are generally quite thick, high etch rates are also desirable, particularly when single wafer processing is being performed (as opposed to batch-type processing) to permit high throughput.
The etch reactor of Ogle, while useful to etch conductive metal layers, cannot meet the above-described etch requirements for oxides such as silicon oxide. In general silicon oxide is etched with fluorine-containing etch gases, as noted above. Silicon oxide is etched with poor selectivity; using gases with a high C:F ratio, or gases containing hydrogen raises selectivity but sacrifices etch rate and produces tapered profiles and microloading. Thus merely increasing the carbon:fluorine ratio of the etch gas, or increasing the gas flow rates, increases the taper of the sidewalls, increases microloading and reduces the etch rate. Fluorohydrocarbon etch gases in addition form polymeric solids that can form particles which deposit on the substrate, causing contamination of the substrate during the etch process.
Thus a means for improving etching of oxide films or layers in the above-described electromagnetically coupled planar plasma equipment would be highly desirable.